Getting a mask one layer to add alpha to another - mask tracking

I'm trying to motion track a solid layer with a mask on it to 'delete' part of a footage layer. I've followed every tutorial I can find, but they always fall short on what i'm looking for.

Till now I have animated masks on the layer I want to add alpha to, but as I want to motion track this alpha mask, I have read you should make it on a separate solid layer above the layer you want to delete from.

I have the mask on the solid motion tracked to the footage, but can't get it to 'delete' the relevant section.

If I 'add' from the mask layer, you just see the solid under the mask

If I then TrkMat to Alpha, the white disappears, but you just see the footage under the mask again